How does multinom() treat NA values by default?

Question

When I am running multinom(), say Y ~ X1 + X2 + X3, if for one particular row X1 is NA (i.e. missing), but Y,

Answer 1

You can specify the behaviour

- na.omit and na.exclude: returns the object with observations removed if they contain any missing values; differences between omitting and excluding NAs can be seen in some prediction and residual functions
- na.pass: returns the object unchanged
- na.fail: returns the object only if it contains no missing values

http://www.ats.ucla.edu/stat/r/faq/missing.htm